remote sensing Article A Real-Time GNSS/PDR Navigation System for Mobile Devices Michele Basso † , Alessio Martinelli † , Simone Morosi *,† and Fabrizio Sera † Department of Information Engineering, University of Florence, via di Santa Marta 3, 50139 Florence, Italy; michele.basso@unifi.it (M.B.); alessio.martinelli@unifi.it (A.M.); [email protected]fi.it (F.S.) * Correspondence: simone.morosi@unifi.it; Tel.: +39-0552758547 † These authors contributed equally to this work. Abstract: In this article, a smart pedestrian navigation system is developed to be implemented in a common smartphone. The main phases that characterize a pedestrian navigation system that is based on dead reckoning are introduced. A suitable Phase-Locked Loop is designed and the algorithm to estimate the direction of the user’s motion between one step and the next is developed. Finally, a suitable multi-rate Kalman filter (KF) is considered to merge the information from the pedestrian dead reckoning (PDR) navigation with the data provided by the global navigation satellite systems (GNSS). The proposed GNSS/PDR navigation system is implemented in Simulink as a finite-state machine and allows to define a trade-off between energy-saving and performance improvement in terms of position accuracy. The presented pedestrian navigation system is independent of the body-worn location of the smartphone and implements a compensation strategy of the systematic errors that are committed on the step-length estimation and the determination of the motion direction. Moreover, several tests are performed by walking in urban and suburban environments: the results show that a suitable trade-off between energy-saving and position accuracy can be reached by switching the GNSS receiver on and off. Citation: Basso, M.; Martinelli, A.; Keywords: PDR; INS; GNSS; pedestrian navigation; sensor fusion Morosi, S.; Sera, F. A Real-Time GNSS/PDR Navigation System for Mobile Devices. Remote Sens. 2021, 13, 1567. https://doi.org/10.3390/ 1. Introduction rs13081567 Nowadays, thanks to the technological development and the mass diffusion of mobile devices, there is an increasing interest in those applications that allow users to keep track Academic Editor: Antonio Vettore of their movements and provide location-based services. Thanks to the development of micro electro-mechanical systems (MEMS) technology, mobile devices are equipped Received: 12 March 2021 with increasingly cheaper and smaller sensors. Positioning and navigation solutions are Accepted: 15 April 2021 generally provided by global navigation satellite systems (GNSS). These systems, together Published: 18 April 2021 with the help of mobile networks, allow to obtain a position solution with bounded errors in a relatively short time, but its prolonged use leads to a rapid drain of the battery and Publisher’s Note: MDPI stays neutral its performance can strongly decrease in those environments that do not present an open with regard to jurisdictional claims in sky visibility of the satellites. This is in contrast to the needs of users, whose interest published maps and institutional affil- is to have both the positioning service also available in harsh environments such as the iations. urban one and a battery life that can cover the whole day. In addition to the GNSS receiver, most portable devices (e.g., smartphones) include sensors such as accelerometer, gyroscope and magnetometer. These additional sensors allow to estimate intensity and orientation of the user’s motion without relying on external infrastructure. By exploiting Copyright: © 2021 by the authors. the computing resources of the processors and the low energy consumption of these sensors, Licensee MDPI, Basel, Switzerland. some manufacturers may also implement additional features in the devices such as fall This article is an open access article detection and step counting, providing useful information on the user’s quality of life [1]. distributed under the terms and In this work, after a review of the state of the art of the pedestrian dead reckoning conditions of the Creative Commons (PDR) navigation techniques, we propose an algorithm for a real time identification of the Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ user’s motion direction by exclusively exploiting the data coming from inertial sensors, 4.0/). and therefore with low energy consumption. We will also show how to smartly merge this Remote Sens. 2021, 13, 1567. https://doi.org/10.3390/rs13081567 https://www.mdpi.com/journal/remotesensing Remote Sens. 2021, 13, 1567 2 of 20 information with GNSS data in order to achieve a twofold objective: increasing the accuracy of the position solution and saving energy at the same time by using the GNSS sensor efficiently. With respect to the state of the art concerning the integration between GNSS and inertial navigation systems [2–4], the present work proposes a real-time implementation of an integrated GNSS/PDR framework which provides several advantages: firstly, a smart trade-off between accuracy and energy consumption; then, the independence from the body-worn location of the smartphone; finally, a compensation strategy of the systematic errors that are committed on the step-length estimation and the determination of the motion direction. Figure1 illustrates the block diagram that highlights the operation of the proposed system: GNSS MARG M Angular Rate Acceleration a g n e t i c fi e l d Device orientation Acc_NED Heading Step Angle Detection PDR/GNSS Integration User Position Figure 1. Block diagram of the operating principle of the proposed GNSS/PDR navigation system. The data coming from the magnetic, angular rate, gravity (MARG) sensor, which is composed of a triaxial accelerometer, gyroscope and magnetometer, is processed by a suitable Madgwick filter in order to obtain the orientation of the mobile device in a fixed North-East-Down (NED) frame; then, the acceleration in NED frame is exploited to identify the single steps, which are used as a trigger signal to drive an algorithm for the estimation of the heading angle of the walk; finally, the data coming from the GNSS is integrated with MEMS data in order to determine the user position. The algorithm for the heading estimation is event-driven, in fact it is essential to identify every single step for determining the direction of advancement of the user’s motion. In Section2 materials and methods used to obtain the proposed GNSS/PDR nav- igation system are illustrated; in particular, the treatment of the methods has been di- vided into the determination of the device orientation (Section 2.2.1), the step detection (Section 2.2.2), the determination of the motion direction (Section 2.2.3), the step length estimation (Section 2.2.5) and the GNSS/PDR integration (Section 2.2.6). Section3 de- scribes and discusses the results obtained; finally, Section4 shows the conclusion of the presented work. Remote Sens. 2021, 13, 1567 3 of 20 2. Materials and Methods This section will describe the hardware and methods which are used to achieve the set goals. 2.1. Materials Mobile devices such as smartphones can differ on technical characteristics including computational power, storage capacity, and available sensors. In this work, a Realme 5 Pro model has been chosen as a mid-range smartphone in order to guarantee the functioning of the implemented system on as many devices as possible. All main technical characteristics of this device are shown in Table1. Table 1. Technical characteristics of the smartphone used to acquire the data. Model Processor Chipset RAM ROM Realme 5 Pro ProKyro 360 Snapdragon 712 4 GB 128 GB The IMU used in this model of smartphone is the BMI160 [5] produced by Bosch. It has been designed for low power, high precision 6-axis applications in mobile phones, tablets, wearable devices, remote controls, game controllers, head-mounted devices and toys. When accelerometer and gyroscope are in full operation mode, power consumption is typically 925 µA, enabling always-on applications in battery driven devices. In this case is a 6-DOF IMU that includes a triaxial accelerometer and gyroscope. Another 3 DOFs are implemented by the triaxial magnetometer that is the AK09918 [6] composed by a triaxial electronic compass IC with high sensitive Hall sensor technology. The Snapdragon 712 chipset of the Realme 5 Pro smartphone allows the access to the following satellite constellations: GPS, GLONASS, BEIDOU, GALILEO, QZSS, and the SBAS. In particular, the GPS, GLONASS, BEIDOU, and GALILEO constellations refer respectively to the American, Russian, Chinese and European global navigation satellite systems; QZSS refers to a regional navigation satellite system aiming to enhance GPS in the Asia-Oceania regions; SBAS refers to the satellite-based augmentation systems aiming to improve the accuracy and the reliability of the GNSS information [7]. An objective of the algorithm that integrates PDR and GNSS is to take rid of the non-stop use of satellite data in order to achieve energy saving. Therefore, it is necessary to choose an appropriate time interval between subsequent activations of the GNSS receiver. In order to obtain comparable data, it was decided to acquire the GNSS data (latitude, longitude, and accuracy associated to the position solution) on a continuous time basis and to implement a control logic to use the data in the algorithm at regular predetermined intervals with a predefined duty cycle. In this way, it was possible to run the GNSS/PDR algorithm multiple times on the same data in order to define the best time interval between subsequent activations of the GNSS receiver. It is necessary to consider that once the satellite receiver is switched on, it will be necessary to wait for a certain time interval to achieve the time to first fix (TTFF), which is a measure of the time required for a GNSS navigation device to acquire the satellite signals, decoding the navigation data, and to calculate a position solution.